Some traditional solar water heating systems use solar collection panels having a series of tubes arranged in parallel through which heat exchange fluid flows. As shown in FIG. 1, the tubes 1 of a solar collection panel 10 may be housed between plates 2, 3 and connected at their ends by manifolds 4, 5. The manifolds 4, 5 serve to distribute heat exchange fluid traveling to the collection panel 10 amongst the several tubes 1 at one end and to collect and carry away heat exchange fluid or water that has passed through the tubes 1 at an opposite end.
This type of solar collection panel typically has relatively poor heat transfer between the plate surface and the heat exchange fluid passing through the tubes. Normally, the tubes must be in contact with the plate surface in some way in order to effect heat transfer. However, because the tubes are typically either cylindrical or “D” shaped, the heat transfer surface between the plate surface and tubes is minimal and consequently a limited amount of heat transfer occurs between the plates and the fluid running through the tubes. To compensate for the limited heat transfer surface area, the solar collection panels can be made larger, but this not only increases the cost and weight of the system but also renders the solar collection panels more difficult to blend into their surroundings.
Other problems with previously known solar water heating systems such as those shown in FIG. 1 stem from the use of heat exchangers for transferring heat from the heat exchange fluid to the water stored in a water storage tank. FIG. 2 shows such a configuration, in which the traditional solar heating systems 20 employs a solar collection panel 21 in fluid communication with a coiled heat exchanger 22 disposed inside a water storage tank 23. One problem with this configuration is that the coiled heat exchanger 22 has a relatively small surface area for transferring heat to water passing around the coiled heat exchanger 22 in the water storage tank 23, reducing efficiency of the solar water heating system. The use of heat exchangers also adds complexity and cost to the system, as well as additional installation and maintenance issues.
These prior art systems also commonly utilize manifolds, such as those shown in the prior art solar collection panel of FIG. 1, for transferring fluid in and out of the solar collection panel. These manifolds are often the source of structural weakness in the solar collection panels. This can be due to the manner in which the manifolds are manufactured, which tends to include the shaping and bending of relatively thin sheets of metal. This can also be due to the silver brazing technique used to secure the manifold to the panel. The silver brazing anneals the manifold and creates structural weakness. Additionally, expansion of the manifolds as a result of heat transfer fluid freezing inside the manifolds can create further structural weaknesses. In some cases, even one freezing and thawing of heat transfer fluid inside the manifolds will ultimately result in the manifolds cracking and ruining the solar collection panel. The use of manifolds also makes it difficult to minimize the amount of space occupied on, for example, the roof of the building where the solar collection panels are installed. The manifolds extend beyond the solar collection panels and require the use of connectors, which means the solar collection panels cannot be arranged so that they abut one another. As a result, it becomes more difficult to blend the solar collection panels into the surrounding environment.
Due to some or all of the issues described above, the operating conditions for these traditional solar water heating systems are usually severely restricted and/or must be closely monitored. Often, substantial effort is undertaken to protect the solar collection panels from freezing conditions. In some cases, additional equipment is incorporated into the solar water heating system to safeguard against the freezing of heat exchange fluid in the solar collection panel. Such equipment (e.g., drain back systems and heat exchangers) can significantly increase the overall cost of the solar water heating system and result in additional maintenance issues.
Some other previously known solar water heating systems use envelope solar collection panels. U.S. Pat. No. 4,285,334 to Collins describes such a system. FIGS. 3 and 4 show a solar water heating system that uses an envelope collection panel 12 having a pair of steel plates 64, 66 seam welded together along the edges 68 and spot welded together at various points 70 to form flow passageways 72 between the plates.
Prior art solar collection panels having an envelope design usually have low tolerance for increased pressure levels. Since municipal water systems tend to operate at such increased pressure levels, prior art envelope solar collection panels usually require apparatus to reduce pressure in the line for water circulating through the envelope solar collection panels. Such equipment is costly and makes the system more complex. The need for additional equipment in such systems also makes installation and maintenance more difficult and expensive.
Envelope solar collection panels also typically require a heat exchanger in order to isolate the envelope solar collection panel from the high pressure inside the water system. As discussed above, heat exchangers increase the installed cost of the solar water heating systems and decrease the efficiency of the system.
Additionally, Collins indicates at, for example, column 4, lines 9-24, that the envelope solar collection panels described therein are not suited for use in extreme temperatures, and systems including envelope solar collection panels must include a drain back mechanism that allows for all fluid in the solar collection panel to be removed from the solar collection panel during freezing conditions.